The invention relates to gas concentrators, and in particular to a concentrator which includes the capability to monitor the pressure of individual adsorbent beds and use the pressure data to track and adjust the concentrator's performance. The application is particularly directed to portable oxygen concentrators for therapeutic use.
The application of oxygen concentrators for therapeutic use is known, and many variants of such devices exist. A particularly useful class of oxygen concentrators is designed to be portable, allowing users to move about and to travel for extended periods of time without the need to carry a supply of stored oxygen. Most of these portable concentrators are based on Pressure Swing Adsorption (PSA) or Vacuum Pressure Swing Adsorption (VPSA) designs which feed compressed air to selective adsorption beds. In a typical oxygen concentrator, the beds selectively adsorb nitrogen, resulting in pressurized, oxygen-rich product gas.
The main elements in an oxygen concentrator are shown in FIG. 1. Air is draw in, and typically filtered, at air inlet 1 before being pressurized by compressor 2. The pressurized air is directed by a valve arrangement through adsorbent beds 3. An exemplary adsorbent bed implementation, used in a concentrator design developed by the inventors, is three columns filled with zeolite powder. The pressurized air is directed through these columns in a series of steps which constitute a PSA cycle. Although many different arrangements of beds are possible as well as a variety of different PSA cycles, the result is that nitrogen is removed by the adsorbent material, and the resulting oxygen rich air is routed to a product gas storage device at 4. Some of the oxygen rich air is routed back through the bed to flush out (purge) the trapped nitrogen to an exhaust. Generally multiple beds, or columns in the exemplary device, are used so at least one bed may be used to make product while at least one other is being purged, ensuring a continuous flow of product gas. The purged gas is exhausted from the concentrator at 6.
Such PSA systems are known in the art, and it is appreciated that the gas flow control through the compressor and the beds in a PSA cycle is complex, and requires precise timing and control of parameters such as pressure and temperature to attain the desired oxygen concentration in the product gas stream. Accordingly, most modern concentrators also have a programmable controller 5, typically a microprocessor, to monitor and control the operation of the PSA cycle and monitor various system parameters to ensure correct device operation. In particular, the controller controls the timing and operation of the various valves used to cycle the beds through feed and purge steps which make up the PSA cycle. Also present in most portable concentrators is a Conserver 7 which acts to ensure that oxygen rich gas is only delivered to a patient when a breath is inhaled, thus using less product than a continuous flow arrangement, thereby allowing for smaller, lighter concentrator design. In the conserver mode, a pulse of oxygen rich air, called a bolus, is delivered in response to a detected breath. A typical concentrator will also contain a user/data interface 8.
A portable oxygen concentrator must be small, light and quiet in order to be useful, while retaining the capacity to produce a flow of product gas, usually a flow rate prescribed by a medical practitioner, adequate to provide for a patient's needs. Although fixed site PSA based concentrators have been available for many years, such fixed site units may weigh 50 pounds or more, be several cubic feet in size and produce sound levels greater than 50 dBA. A portable concentrator on the other hand may weigh on the order of 10 lbs, be less than one half cubic foot in size and produce as little as 35-45 dbA sound levels. The portable concentrators still need to produce the prescribed flow rate of oxygen. Thus portable concentrators involve a significant amount of miniaturization, leading to smaller, more complex designs compared to stationary units. System size, weight and complexity may lead to minimum design margins and tighter manufacturing tolerances to achieve portability.
Therefore it is important that portable concentrator performance is monitored closely, so problems can be compensated for quickly without degrading oxygen service to the patient which could leave patients stranded without an adequate oxygen supply. A key parameter which affects concentrator performance is the balance of the pressure ratio achieved in each bed during the PSA cycle. The purity of the oxygen product depends greatly on the operation of the columns being well balanced in operating parameters; that is, if one column has a slightly longer adsorption step or a higher adsorption pressure ratio or likewise a longer blowdown or countercurrent purge step than the other due to dissimilar valve response times or other tolerance stack-up issues, a significant decrease in product purity will result. Subtle differences in purge step symmetry caused by many different factors other than step timing can also lead to a decrease in product purity if left uncorrected.
Typical home oxygen concentrators are not designed to have the capability to make subtle PSA cycle parameter changes. Prior art home oxygen concentrators typically utilize a single feed/waste valve assembly that does not allow for independent adjustment of step times. These devices often utilize check valves in place of solenoid valves to control product gas flow, and they use a single fixed orifice to control the purge rate. The PSA cycle steps are all controlled by the single switching of the main feed/waste valve assembly, so the step times and the flow rates are symmetrically fixed for all adsorbent beds in the system. This hardware configuration causes pressure balance asymmetries to be ignored in a stationary concentrator's manufacture and operation.
Industrial scale PSA systems that utilize up to seven large adsorbent beds typically have hardware means to adjust bed parameters individually since the operating cost of an inefficient plant far outweighs the capital cost of adding sensors and controls to the system. The preferred method for balancing PSA bed operation is by monitoring the effluent concentration of the bed during the purge to ensure that the bed is being properly regenerated. While this method enables precise balancing of the bed's oxygen production purity, the equipment required to perform this monitoring would be far too costly and cumbersome to implement in a small portable device. The portable concentrator must be as reliable as the industrial PSA plant even though it does not have an equivalent sensor array or routine maintenance.
In practical use, many things can contribute to symmetry imbalance of the system. In portable lightweight devices, the scale of all parts and adsorbent bed sizes required becomes small, making manufacturing tolerances of parts, such as valves, adsorbent loading (mass or packing factor differences between columns), and assembly critical to proper operation of the PSA cycle. Portable concentrators are also cost sensitive so adding additional sensors or hardware control mechanisms is not advantageous as it is in industrial scale applications. During normal patient use the device will encounter significant levels of shock and vibration which can lead to small leaks in the system that if adjusted for properly could enable the device to continue to operate normally. It is desirable to account for these possibilities in a real-time manner for mass-produced portable devices. The capability to monitor and adjust adsorbent bed pressure accounts for these possibilities, therefore maintaining concentrator reliability and minimizing required maintenance.